TWIM0 not starting in custom driver

RE: TWIM0 not starting in custom driver

PaulJ — Fri, 14 Nov 2025 23:27:58 GMT

OK, some humble pie for me....

Firstly, I did a poor job of comparing my code to the bare metal code that Sigurd provided. My biggest error was not noticing that the "NRF_TWIM0->ENABLED" is set to ON with a "6", not a "1". The second error(s) were the "SHORTS" and "EVENTS". I clearly did not fully understand the logic of that system! So now I have a system that seems to be writing and reading to my peripheral. The numbers look goofy right now but I'm sure that's a "ME" problem (again).

RE: TWIM0 not starting in custom driver

PaulJ — Fri, 14 Nov 2025 04:13:02 GMT

After much editing of various config files and finding a bad connection on my usb cable, I now am getting a "NACK" error recorded on the TWIM registers. Since this is a brand new PCB that I manually assembled, there is a good chance that the IC connected to the I2C bus is not soldered properly so I will try fixing that next and report back if it completely fixes the issue.

RE: TWIM0 not starting in custom driver

PaulJ — Thu, 30 Oct 2025 22:20:09 GMT

A further update: I did see that they recommend that you explicitly clear certain registers in the peripherals that share the same base address as TWIM0. I double checked, and none of the conflicting registers are enabled, have any PPI, or have any interrupts set.

RE: TWIM0 not starting in custom driver

PaulJ — Thu, 30 Oct 2025 20:20:46 GMT

I added the two external pullups (4K) and retested. Still no success in starting the TWIM. I looked at the pin states after I configured the GPIO and they are both showing as "INPUT" and "HIGH". However, as soon as I execute:

NRF_TWIM0->ENABLE = 1;

both pins states go low! It seems to me that the TWIM system should maintain these as "HIGH" when no data is being transferred. Is that any sort of clue as to what is set up incorrectly?

RE: TWIM0 not starting in custom driver

PaulJ — Thu, 30 Oct 2025 16:11:29 GMT

I thought that I could use the internal pullup resistors for the TWI system so I did not include pullups on my custom PCB. I can see now that the "pullup" is only for the "input" configuration. Would the lack of external pullups prevent the TWM from setting its flags as I described above? I will add them now (somehow) and report back but added this interim comment in case people are searching for a similar issue.

RE: TWIM0 not starting in custom driver

PaulJ — Wed, 29 Oct 2025 16:12:29 GMT

Thanks for providing the Nordic bare metal version of the TWIM code. I went through it all and really the only difference between my version and yours is that the "SHORT" is set each time for the read or write whereas I was configuring both SHORTS in my init function. Obviously the physical pins used, the device address, and buffers are different in my version so I used the values suitable to my hardware. Also, your code sets it up for 400KHz while I was trying for 100Khz (at least initially anyway).

I'm afraid the results are the same; the control registers for the TWIM all appear to be set properly (ENABLE = 1, MAXCNT = non zero, DATA_PTR set to RAM location, PSEL = my pin numbers, FREQ = 400khz, DEV_ADDR = my device address. When I execute "NRF_TWIM0->TASKS_STARTTX = 1;", absolutely nothing changes in the system registers so the end condition "while (NRF_TWIM0->EVENTS_STOPPED == 0);" never exits.

Are there any other hardware systems in the nrf52840 that might interfere with this module? Could there be an issue by using the single step in debug mode?

RE: TWIM0 not starting in custom driver

Sigurd — Wed, 29 Oct 2025 13:40:04 GMT

Hi!

I strongly recommend using the provided libraries and drivers

That said, if you want to try something very bare-metal, try something like this:

#include <nrf.h>

#define DEVICE_ADDRESS (0x34)
#define PIN_SCL        (27)
#define PIN_SDA        (26)

void i2c_write(uint8_t addr, uint8_t data)
{
  uint8_t tx_buf[2];
  NRF_TWIM0->SHORTS = TWIM_SHORTS_LASTTX_STOP_Msk;

  tx_buf[0] = addr;
  tx_buf[1] = data;
  NRF_TWIM0->TXD.MAXCNT = sizeof(tx_buf);
  NRF_TWIM0->TXD.PTR = (uint32_t)&tx_buf[0];

  NRF_TWIM0->EVENTS_STOPPED = 0;
  NRF_TWIM0->TASKS_STARTTX = 1;
  while (NRF_TWIM0->EVENTS_STOPPED == 0);
}

uint8_t i2c_read(uint8_t addr)
{
  uint8_t tx_buf[1];
  uint8_t rx_buf[1];
  NRF_TWIM0->SHORTS = TWIM_SHORTS_LASTTX_STARTRX_Msk | TWIM_SHORTS_LASTRX_STOP_Msk;

  tx_buf[0] = addr;
  NRF_TWIM0->TXD.MAXCNT = sizeof(tx_buf);
  NRF_TWIM0->TXD.PTR = (uint32_t)&tx_buf[0];

  NRF_TWIM0->RXD.MAXCNT = 1;
  NRF_TWIM0->RXD.PTR = (uint32_t)&rx_buf[0];

  NRF_TWIM0->EVENTS_STOPPED = 0;
  NRF_TWIM0->TASKS_STARTTX = 1;
  while (NRF_TWIM0->EVENTS_STOPPED == 0);

  return rx_buf[0];
}

int main(void)
{
  volatile uint8_t data;

  NRF_TWIM0->PSEL.SCL = PIN_SCL;
  NRF_TWIM0->PSEL.SDA = PIN_SDA;

  NRF_TWIM0->ADDRESS = DEVICE_ADDRESS;
  NRF_TWIM0->FREQUENCY = TWIM_FREQUENCY_FREQUENCY_K400 << TWIM_FREQUENCY_FREQUENCY_Pos;
  NRF_TWIM0->SHORTS = 0;

  NRF_TWIM0->ENABLE = TWIM_ENABLE_ENABLE_Enabled << TWIM_ENABLE_ENABLE_Pos;

  i2c_write(0x0, 0xFF);

  data = i2c_read(0x0);
  data; // to get rid of set but unused warning

  while (1)
  {
    __WFE();
  }
}